From: Nux! nux@li.nux.ro
As far as I know both Centos 5 and 6 support TRIM ("discard" option in fstab), but only for EXT4 filesystems (and probably XFS).
I do not think CentOS 5 supports TRIM (unless back-ported from 2.6.33)... http://kernelnewbies.org/Linux_2_6_33#head-b9b8a40358aaef60a61fcf12e90559007...
JD